When we look at religion we have to look at the foundational elements. If the foundational elements weren’t there then the religion wouldn’t exist. When we look at Islam, Islam starts and ends with the Shahadah.
Islam revolves around the oneness of God and what we say and do should embrace that. God is unique in every element and He deserves the respect for those elements. When looking at why one should embrace Islam it is to show the reverence of God.
When we ask why should we take the shahadah, it is so people can establish a foundation to be enriched with Allah and His Oneness. We need a foundation to make us a principled person of conviction.